1. Field of the Invention
The present invention relates to a device for use in a laser printer using a polygon mirror for light spot scanning on a photosensitive member and more particularly to a frequency converter for use in changing clock frequency for controlling print data speed corresponding to the change of the light spot speed due to change of the light projection angle of the polygon mirror.
2. Description of the Prior Art
In the laser printer, as shown in FIG. 1, a laser diode 1 is intermittently driven to emit pulsive laser light rays corresponding to the printing data and the pulsive laser light rays are focused onto a cylindrical photosensitive member 3 through a collimating lens 2. A rotating polygon mirror 4 and f-.theta. lens are disposed between the collimating lens 2 and the photosensitive member 3. The polygon mirror 4 causes the laser light beam to scan on the photosensitive member 3 in a rapid speed. The laser diode 1 is turned on and off corresponding to the rotation of a motor (not shown) which rotates the polygon mirror 4 so that desired parts of the surface of the photosensitive member 3 can be exposed by the laser light beam.
The f-.theta. lens 5 is provided by the following reason. As the rotation speed of the polygon mirror is kept constant by the motor (not shown), the light spot speed of the laser light on the photosensitive member 3 is faster away from the center towards the both side ends than the speed at the center of the photosensitive member 3, whereby the dot pitch becomes larger on both sides in the case that the printing data is applied at a constant speed. In order to make the calculation, it is assumed that a rotation plane mirror 6 is used in place of using the polygon mirror 4. It is noted that the error due to the replacement mirror is small. Assuming that the angle between the optical axis of the laser diode 1 and the plane mirror 6 is .theta.+45.degree., the distance x from the center S of the photosensitive member 3 to the spot position P is x=l tan 2.theta.. Wherein .theta.=wt, w; angular speed of the plane mirror 6. Accordingly the light spot speed v on the photosensitive member 3 is EQU v(t)=dx/dt=2wl sec.sup.2 (2wt).
The light spot speed rate is EQU [v(t)vt=0]=sec.sup.2 (2wt).
The relation between the spot position and the light spot speed rate can be depicted in FIG. 3.
In order to correct the distortion of f (focus distance)-.theta.(incident angle), the f-.theta. lens 5 is disposed between the rotation polygon mirror 4 and the photosensitive member 3. The f-.theta. lens 5 has such a characteristic that the light beam tends to be displaced inwardly as the light goes to the periphery of the mirror 4 so that the light beam from the laser diode 1 is focused on the photosensitive member 3 and the scanning distance is proportional to the scanning time.
The f-.theta. lens must have a specialized expensive structure which causes the cost of the laser printer to be expensive.
Therefore, it is strongly desired to provide an inexpensive f-.theta. correcting device.
One approach is to correct the f-.theta. distortion by changing the clock frequency of the pulses used for controlling the speed of the printing data corresponding to the light spot speed as shown in FIG. 3.
One known example device for the above purpose is shown in FIG. 4, wherein a clock pulse oscillator 7 and a frequency divider 8 ae used so that the frequency of the clock pulse of the oscillator 7 is divided and changed in the frequency divider 8 1% by 1%. The frequency divider 8 consists of seven stage flip flops so as to obtain eight kinds of frequency dividing ratios of 1/128 to 1/121 in response to the frequency change data signal applied to the terminals A, B and C, thereby producing the printing data clock pulse having desired clock frequencies from the terminal QG. However, in the conventional laser printer, a printing clock pulse of about 10 MHz is used, so that the pulse frequency of the oscillator 7 must be 1.28 GHz. However such high speed logic devices are not commercially available and therefore, the f-.theta. correction arrangement mentioned above is not available for the practical use.